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[PATCH] [BZ #18875] Avoid excess precision in check against float zero

From: "H.J. Lu" <hjl dot tools at gmail dot com>
To: Carlos O'Donell <carlos at redhat dot com>
Cc: GNU C Library <libc-alpha at sourceware dot org>, "Joseph S. Myers" <joseph at codesourcery dot com>
Date: Wed, 26 Aug 2015 06:37:45 -0700
Subject: [PATCH] [BZ #18875] Avoid excess precision in check against float zero
Authentication-results: sourceware.org; auth=none
References: <559C2EA4 dot 8030409 at redhat dot com>
On Tue, Jul 07, 2015 at 03:55:16PM -0400, Carlos O'Donell wrote:
> Joseph,
> 
> I'm seeing these failures on i686 in test-float, have you seen these?
> If not, I'll investigate the failures to see if they are related to
> my compiler or linker.
> 
> glibc-i686/math/test-float.out:
> 
> testing float (without inline functions)
> Failure: exp10 (-0x1.31p+8): errno set to 0, expected 34 (ERANGE)
> Failure: exp10 (-0x1.343792p+12): errno set to 0, expected 34 (ERANGE)
> Failure: exp10 (-0x1.343794p+12): errno set to 0, expected 34 (ERANGE)
> Failure: exp10 (-0x1.344p+12): errno set to 0, expected 34 (ERANGE)
> Failure: exp10_downward (-0x1.31p+8): errno set to 0, expected 34 (ERANGE)
> Failure: exp10_downward (-0x1.343792p+12): errno set to 0, expected 34 (ERANGE)
> Failure: exp10_downward (-0x1.343794p+12): errno set to 0, expected 34 (ERANGE)
> Failure: exp10_downward (-0x1.344p+12): errno set to 0, expected 34 (ERANGE)
> Failure: exp10_towardzero (-0x1.31p+8): errno set to 0, expected 34 (ERANGE)
> Failure: exp10_towardzero (-0x1.343792p+12): errno set to 0, expected 34 (ERANGE)
> Failure: exp10_towardzero (-0x1.343794p+12): errno set to 0, expected 34 (ERANGE)
> Failure: exp10_towardzero (-0x1.344p+12): errno set to 0, expected 34 (ERANGE)
> Failure: exp2 (-0x1.3045fep+8): errno set to 0, expected 34 (ERANGE)
> Failure: exp2 (-0x3.fb8p+8): errno set to 0, expected 34 (ERANGE)
> Failure: exp2 (-0x3.fc8p+8): errno set to 0, expected 34 (ERANGE)
> Failure: exp2 (-0x3.fd8p+8): errno set to 0, expected 34 (ERANGE)
> Failure: exp2 (-0x4.01p+12): errno set to 0, expected 34 (ERANGE)
> Failure: exp2 (-0x4.32p+8): errno set to 0, expected 34 (ERANGE)
> Failure: exp2_downward (-0x1.3045fep+8): errno set to 0, expected 34 (ERANGE)
> Failure: exp2_downward (-0x3.fb8p+8): errno set to 0, expected 34 (ERANGE)
> Failure: exp2_downward (-0x3.fc8p+8): errno set to 0, expected 34 (ERANGE)
> Failure: exp2_downward (-0x3.fd8p+8): errno set to 0, expected 34 (ERANGE)
> Failure: exp2_downward (-0x4.01p+12): errno set to 0, expected 34 (ERANGE)
> Failure: exp2_downward (-0x4.32p+8): errno set to 0, expected 34 (ERANGE)
> Failure: exp2_towardzero (-0x1.3045fep+8): errno set to 0, expected 34 (ERANGE)
> Failure: exp2_towardzero (-0x3.fb8p+8): errno set to 0, expected 34 (ERANGE)
> Failure: exp2_towardzero (-0x3.fc8p+8): errno set to 0, expected 34 (ERANGE)
> Failure: exp2_towardzero (-0x3.fd8p+8): errno set to 0, expected 34 (ERANGE)
> Failure: exp2_towardzero (-0x4.01p+12): errno set to 0, expected 34 (ERANGE)
> Failure: exp2_towardzero (-0x4.32p+8): errno set to 0, expected 34 (ERANGE)
> Failure: pow10 (-0x1.31p+8): errno set to 0, expected 34 (ERANGE)
> Failure: pow10 (-0x1.343792p+12): errno set to 0, expected 34 (ERANGE)
> Failure: pow10 (-0x1.343794p+12): errno set to 0, expected 34 (ERANGE)
> Failure: pow10 (-0x1.344p+12): errno set to 0, expected 34 (ERANGE)
> Failure: pow10_downward (-0x1.31p+8): errno set to 0, expected 34 (ERANGE)
> Failure: pow10_downward (-0x1.343792p+12): errno set to 0, expected 34 (ERANGE)
> Failure: pow10_downward (-0x1.343794p+12): errno set to 0, expected 34 (ERANGE)
> Failure: pow10_downward (-0x1.344p+12): errno set to 0, expected 34 (ERANGE)
> Failure: pow10_towardzero (-0x1.31p+8): errno set to 0, expected 34 (ERANGE)
> Failure: pow10_towardzero (-0x1.343792p+12): errno set to 0, expected 34 (ERANGE)
> Failure: pow10_towardzero (-0x1.343794p+12): errno set to 0, expected 34 (ERANGE)
> Failure: pow10_towardzero (-0x1.344p+12): errno set to 0, expected 34 (ERANGE)
> 
> Test suite completed:
>   38654 test cases plus 36610 tests for exception flags and
>     36610 tests for errno executed.
>   42 errors occurred.
> 

For check against float zero:

  float z;
  z = ...
  if (z == 0)

on x86 with excess precision, we may get

(gdb) info float
=>R7: Valid   0x3d30e0d9b4d78a806800 +6.369668676344082156e-217
  R6: Empty   0x40029000000000000000

and z (+6.369668676344082156e-217) == 0 is false.  We can avoid excess
precision by checking binary representations of float zero directly.

OK for master?

Thanks.

H.J.
---
	[BZ #18875]
	* sysdeps/i386/fpu/w_exp10f.c: New file.
	* sysdeps/i386/fpu/w_exp2f.c: Likwise.
	* sysdeps/i386/fpu/w_expf.c: Likwise.
---
 sysdeps/i386/fpu/w_exp10f.c | 41 +++++++++++++++++++++++++++++++++++++++++
 sysdeps/i386/fpu/w_exp2f.c  | 38 ++++++++++++++++++++++++++++++++++++++
 sysdeps/i386/fpu/w_expf.c   | 37 +++++++++++++++++++++++++++++++++++++
 3 files changed, 116 insertions(+)
 create mode 100644 sysdeps/i386/fpu/w_exp10f.c
 create mode 100644 sysdeps/i386/fpu/w_exp2f.c
 create mode 100644 sysdeps/i386/fpu/w_expf.c

diff --git a/sysdeps/i386/fpu/w_exp10f.c b/sysdeps/i386/fpu/w_exp10f.c
new file mode 100644
index 0000000..bee27b0
--- /dev/null
+++ b/sysdeps/i386/fpu/w_exp10f.c
@@ -0,0 +1,41 @@
+/* Copyright (C) 2015 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+
+/*
+ * wrapper exp10f(x)
+ */
+
+#include <math.h>
+#include <math_private.h>
+
+float
+__exp10f (float x)
+{
+  float z = __ieee754_exp10f (x);
+  int32_t iz;
+  GET_FLOAT_WORD (iz, z);
+  if (__builtin_expect (!isfinite (z) || (iz & 0x7fffffff) == 0, 0)
+      && isfinite (x) && _LIB_VERSION != _IEEE_)
+    /* exp10f overflow (146) if x > 0, underflow (147) if x < 0.  */
+    return __kernel_standard_f (x, x, 146 + !!signbit (x));
+
+  return z;
+}
+weak_alias (__exp10f, exp10f)
+strong_alias (__exp10f, __pow10f)
+weak_alias (__pow10f, pow10f)
diff --git a/sysdeps/i386/fpu/w_exp2f.c b/sysdeps/i386/fpu/w_exp2f.c
new file mode 100644
index 0000000..bf86212
--- /dev/null
+++ b/sysdeps/i386/fpu/w_exp2f.c
@@ -0,0 +1,38 @@
+/* Copyright (C) 2015 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+/*
+ * wrapper exp2f(x)
+ */
+
+#include <math.h>
+#include <math_private.h>
+
+float
+__exp2f (float x)
+{
+  float z = __ieee754_exp2f (x);
+  int32_t iz;
+  GET_FLOAT_WORD (iz, z);
+  if (__builtin_expect (!isfinite (z) || (iz & 0x7fffffff) == 0, 0)
+      && isfinite (x) && _LIB_VERSION != _IEEE_)
+    /* exp2 overflow: 144, exp2 underflow: 145 */
+    return __kernel_standard_f (x, x, 144 + !!signbit (x));
+
+  return z;
+}
+weak_alias (__exp2f, exp2f)
diff --git a/sysdeps/i386/fpu/w_expf.c b/sysdeps/i386/fpu/w_expf.c
new file mode 100644
index 0000000..5eb7f56
--- /dev/null
+++ b/sysdeps/i386/fpu/w_expf.c
@@ -0,0 +1,37 @@
+/* Copyright (C) 2015 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <math.h>
+#include <math_private.h>
+
+/* wrapper expf */
+float
+__expf (float x)
+{
+  float z = __ieee754_expf (x);
+  int32_t iz;
+
+  GET_FLOAT_WORD (iz, z);
+
+  if (__builtin_expect (!isfinite (z) || (iz & 0x7fffffff) == 0, 0)
+      && isfinite (x) && _LIB_VERSION != _IEEE_)
+    return __kernel_standard_f (x, x, 106 + !!signbit (x));
+
+  return z;
+}
+hidden_def (__expf)
+weak_alias (__expf, expf)
-- 
2.4.3
Follow-Ups:
- Re: [PATCH] [BZ #18875] Avoid excess precision in check against float zero
  - From: Joseph Myers
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